Solenoid valves are, of course, well known. In general, these devices have a body, a plunger mounted on the body for movement toward and away from a seat, a coil mounted on the body and surrounding the plunger, and a return spring acting between the body and plunger for continuously urging the plunger to move toward the seat. The coil is adapted to be supplied with an electrical current for selectively displacing the plunger away from the seat. The seat may typically surround an inlet opening, and the plunger may be force-balanced against variations in the inlet pressure.
Such solenoid valves are basically on-off devices. In other words, when the coil is de-energized, the return spring will move the plunger to engage the seat, thereby preventing flow through the valve. On the other hand, when the coil is energized, the plunger will be selectively displaced off the seat to a fully-opened position so as to permit flow through the valve.
Some solenoid valves employ centering springs to bias the plunger to a predetermined de-energized position relative to a body. By using two coils (i.e., a coil on either side of such spring-centered position) and by selectively energizing the appropriate coil, the plunger may be selectively displaced in the desired direction from such spring-centered position.
It is also known to operate a solenoid valve by a pulse-width-modulated technique. Basically, a train of command current pulses is provided to the coil at a predetermined repetition rate, and the percentage of time that the valve is open is a function of the pulse widths. By using this technique, the valve is caused to repeatedly open and close many times per second, to provide controlled flow through the valve. However, the use of such PWM-operated solenoid valves suffers from the disadvantage that the valve does not open simultaneously with the leading edges of the supplied current pulses. Applicant's experience has demonstrated that there is an initial "deadzone" in the operation of such devices. This "deadzone", is attributable to a lag in the dynamic response of the valve to the supplied current pulses.
On the other hand, an electrohydraulic servovalve is a widely-used device for providing a hydraulic output in response to an input electrical signal. In a flow-control servovalve, such as shown and described in U.S. Pat. No. 3,023,782, the aggregate disclosure of which is hereby incorporated by reference, the output flow of the valve is substantially proportional to the magnitude and polarity of the input current. Other types of proportional valves (e.g., two-way, three-way and four-way) are also known. In some of these, the amount of valve opening is proportional to the magnitude of an input electrical current, as determined by applying electromagnetic force to a spring-restrained valve element.
It has been proposed to use a pulse-width-modulated solenoid valve to control the flow of gas with respect to an actuator, as an alternative to a conventional servovalve or some other proportional-type valve. See, e.g., Thayer, "Electropneumatic Servoactuation--An Alternative to Hydraulics for Some Low Power Applications", Technical Bulletin 151, Moog Inc. (1984). The motivation for this alternative arrangement is largely stimulated by a difference in costs and economics, as opposed to performance. However, the presence of the "deadzone" or lag in the dynamic response characteristics of such PWM-controlled solenoid valves, has impeded their use as alternatives to conventional servovalves or proportional-type valves, particularly when it is desired to incorporate the same into a closed-loop servosystem. In effect, the presence of the "deadzone" causes the PMW-controlled solenoid valves to have a variable or non-proportional gain (i.e., output/input) which seriously degrades the static accuracy and low amplitude/low frequency dynamic response of the servosystems in which such valves are incorporated.